tests: json: add missing/expected json output

tests: json: add missing/expected json output

nft-test.py generates following warning:
any/last.t: WARNING: line 12: '{"nftables": [{"add": {"rule": {"family": "ip", "table": "test-ip4", "chain": "input", "expr": [{"last": {"used": 300000}}]}}}]}': '[{"last": {"used": 300000}}]' mismatches '[{"last": null}]'

This is because "last" expression is stateful; but nft-test.py
explicitly asks for stateless output.

Thus we need to provide a json.output file, without it,
nft-test.py uses last.json as the expected output file.

Fixes: ae8786756b0c ("src: add json support for last statement")
Signed-off-by: Florian Westphal <fw@strlen.de>